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Updated Reminder about DST Asymmetry

It's March: time for my annual reminder about the asymmetry of daylight savings time. If you've seen this before, well, here's a reminder. This time, there's a bonus: a Google App Engine application that'll make a nice graph.

The summary: Daylight Savings Time is not symmetrical around the solar calendar — it does not "surround" the days of longest light as one might expect if one hasn't thought about it. So if you like to do outdoors stuff (e.g., hiking or mountain-bike riding) while it's light out, especially mid-week after work, the time to get started with that sort of outdoors stuff is NOW.

Some details: If we were to start DST at the spring equinox and end it at the autumn equinox, then it would be close to symmetric in the sense I'm talking about. Around the day we spring ahead into daylight savings time, sunset would change from being (say) 6:10 to being 7:10; and the day we fall back to winter time, it would go from 7:10 back to 6:10.
But we don't do the DST changes symmetrically around the equinoxes: we spring ahead in early March, roughly one or two weeks before the after the spring equinox, and we fall back in early November, around six or seven wo weeks after the fall equinox. This means that it's lighter later into each day at the start of DST than it is at the end. In fact, a little bit of trigonometry (or some actual observation) will tell you that the equinoxes are the times of the fastest change in the days' lengths, so it's lighter a lot later at the start of DST than at the end.

Around here (San Francisco Bay Area), the springtime change this year (2009) made our notion of sunset change from 6:10pm to 7:10pm. So it's already worth getting out for a short (60- to 90-minute) hike or ride after work even if you have to work until 6:00pm.

By contrast, the autumn change around here, this year, makes our label for "sunset-time" change from 6:10pm to 5:10pm. So that's a full hour's difference, compared to the springtime change. It generally means that despite the usually dry autumn weather we get, those of us who have to work until 6:00pm or later have few choices by October — learn to like riding with lights (and to find places where it's legal), or stick to riding or hiking only on weekends, "hookey" weekdays, or long "lunch"es.

Bottom line: For those of us for whom snow is not an issue, the dry parts of March or April are the time to start those after work hikes or rides.

So after years of writing the occasional exhortation/warning about this, I finally wrote an app engine app to display the sunrise and sunset data graphically, so you could more easily eyeball and internalize the asymmetry. Here's the link: DST Asymmetry. It's somewhat fragile at the moment, and still has a couple of extraneous debugging links and a css that, um, could be nicer.

Finally, here are links to a couple of earlier versions of this warning: Seven years ago; and Two years ago.